Particulate monitoring in a clean environment is a crucial operation in semiconductor fabrication facilities. Particulate contamination causes many problems in the semiconductor manufacturing process. For example, yields may be significantly reduced by the presence of contaminants present on the surface of the wafer during manufacturing. If the number and size of airborne particles can be quantified, then a determination can be made as to the effect of different sized particles on the semiconductor process.
Current methods of counting airborne particles include laser particle counters (LPCs), scanning surface inspection systems (SSISs) for scanning wafers, and condensation nucleus counters (CNCs). The LPC is a laser based optical particle counter. A particle travels through a focused laser beam which produces scattered light at the detector which is converted to an electrical signal. The LPC typically is useful only for sizes larger than 0.200 .mu.m. Newer LPCs, however, are capable of detecting particles to less than 0.100 .mu.m, but they are bulkier and are more costly to acquire. Moreover, even the newer LPCs will not suffice for next generation semiconductor processes which will achieve submicron line widths, where particle sizes on the order of 0.050 .mu.m will be significant.
Surface inspection systems for wafer scanning are expensive. They tend to be impractical in a manufacturing environment due to the long data collection times necessary to adequately collect particle information for the wafer. In addition, there are problems with skewed data results, since larger particles on the wafer tend to be counted with greater efficiency than smaller particles. The distribution of particle sizes is further skewed if the effects of the impaction of larger particles are taken into account. The smaller particles are kept off the surface by a hydrodynamic boundary layer and therefore follow the streamlines around the wafer.
A condensation nucleus counter offers good low end particle resolution, and at the same time can be used for particle sizes larger than 3 .mu.m. The principle behind a CNC is the same as the LPC, but the particles in a CNC are first subjected to a heterogeneous condensation step, whereby the particles are "grown" to sizes on the order of 5-7 .mu.m. By so doing, the particle diameters are large enough to be efficiently counted.
For a given saturation ratio, the vapor can only condense onto a particle which is large enough to serve as a nucleation point. The degree of supersaturation is measured by its saturation ratio. The saturation vapor pressure is defined as the equilibrium partial pressure for a liquid surface at a given temperature. This minimum particle size acting as a nucleation source is called the Kelvin diameter, and is shown by the relationship: ##EQU1## where: P is the vapor pressure of the condensing fluid;
P.sub.s is the saturation vapor pressure at temperature, T; PA1 g is the surface tension of the condensing fluid; PA1 M is the molecular weight of the condensing fluid; PA1 r is the density of the condensing fluid; PA1 R is the universal gas constant; PA1 T is the absolute temperature; and PA1 d is the Kelvin diameter.
The Kelvin diameter is significant only for particles less than 0.1 .mu.m. The larger the saturation ratio, the smaller the Kelvin diameter. A large saturation ratio will also cause the particle to grow. The above equation is important since the saturation ratio is dependent upon the differential setpoint temperature. For every droplet size, there is one saturation ratio that will exactly maintain that size particle; too great a saturation ratio and the particle grows; too small and it evaporates. Conversely, for a given saturation ratio only those particles having a certain diameter (d) are stable; smaller particles evaporate, larger particles grow.
Basically, a CNC consists of a saturator containing a bath of condensing fluid, through which an air sample is passed. The air sample, being saturated with vapors from the bath, is then passed through a condenser. By setting the temperature of the condenser to be less than that of the saturator, the vapors in the saturated air sample condense onto individual particles suspended in the air sample, thus enlarging the particles to form droplets. The droplets are then detected by optical means, and counted. The temperature differential between the saturator and the condenser is referred to as the differential setpoint temperature.
While CNCs are capable of detecting particles as small as 0.02 .mu.m, the droplets which form in the condensation chamber generally are uniform in size regardless of the sizes of the particles. Thus, it is not possible to ascertain the sizes of the original particles. Nevertheless, it may be desirous to change the detection limit; for example, to exclude the smallest particles (&lt;0.05 .mu.m) which are not yet a concern in semiconductor fabrication. To achieve a different detection limit, external components are relied upon. Two widely used components are diffusion batteries and inertial impactors, which serve as particle size selectors.
A diffusion battery consists of a series of fine meshed screens contained in a housing that determines the threshold size of the CNC (liken to an electrical high pass filter). The diffusion battery is placed in the aerosol inlet of the CNC to raise the lower limit of detectable particle size. The diffusion battery separates particles according to their diffusion coefficients. It eliminates small particles by Brownian motion and passes the larger particles. Besides requiring periodic maintenance, the efficiency curve as a function of particle size is not very steep.
Inertial impactors are another source for size discrimination, but are used for eliminating larger sized particles. A shortcoming of inertial impactors is particle bounce, wherein particles bounce off an impaction stage and get re-entrained in the flow stream. Particle bounce also increases as a function of flow rate for a given particle size. A method of minimizing particle bounce is to apply a layer of grease on the impaction stage. This approach, however, is not suited for the ultra-clean environments required of semiconductor fabrication facilities. An inertial impactor can be used in combination with a diffusion battery to operate a CNC for detection of particles within a range of sizes, but with the above-described limitations.
The setpoint temperature of conventional CNCs is maintained by measuring the actual temperature differential between the saturator and condenser. The temperature of the saturator and/or the condenser is then adjusted by an amount directly proportional to the amount of deviation from the setpoint temperature. For example, commercially available CNCs typically employ a thermoelectric device to transfer heat from the condenser to the saturator. The setpoint is maintained first by determining the difference between the measured differential temperature and the setpoint temperature. This difference, referred to as an error term, may be used directly as a control signal to operate the thermoelectric device. Alternatively, the error term may be reduced by a certain amount, e.g. divided by a constant factor, to obtain the correction signal.
The foregoing methods of maintaining the setpoint temperature lead to oscillations in the actual temperature differential (.DELTA.T) between the saturator and condenser, as can be seen in the graph of FIG. 3. It has been observed that variations in the actual temperature differential can be greater than .+-.1.5.degree. C. about the setpoint temperature.
It has been further observed that when the .DELTA.T fluctuates, so does the number of particles counted. This is dramatically illustrated in the graph of FIG. 3, showing both the .DELTA.T fluctuations and the particle count variations over time. The .DELTA.T is shown by a solid line, and the particle count is shown by a dotted line. Data for the graph was taken after the CNC reached steady state conditions, roughly fifty minutes, as shown by the time scale. The particle source consisted of 0.100 .mu.m particles at a constant aerosol concentration of 100 particles per second detected at varying .DELTA.Ts.
When the .DELTA.T varies by plus or minus one degree from a setpoint of 12.degree. C., the particle counts vary from about zero (0) particles counted per second to 500 particles counted per second. Such variations in counted particles pose a tremendous problem in ascertaining an accurate level of air purity in a modern clean room.
Interestingly enough, there is no indication in the prior art which recognizes the problem of temperature fluctuations about the setpoint temperature. Prior art patents relating to CNC technology relate to CNC devices, improvements in the hardware, and enhanced functionality; and not to performance of the CNC. For example:
U.S. Pat. Nos. 3,806,248, 4,293,217, and 4,790,650 each discloses a portable CNC capable of providing continuous on-line operation.
U.S. Pat. No. 4,128,335 relates to a CNC apparatus for counting particles by taking multiple measurements and discarding those measurements in excess of an arbitrarily selected value.
U.S. Pat. No. 4,449,816 describes a continuous operation CNC featuring a design that avoids aerosol particle deposition on the wall surface of the cooling nozzle.
U.S. Pat. No. 4,792,199 discloses a CNC capable of monitoring extremely small particles in low pressure environments in real-time.
U.S. Pat. Nos. 4,950,073 and 5,026,155 each relates to an apparatus for counting particles of different sizes.
U.S. Pat. No. 5,118,959 discloses a CNC capable of avoiding water contamination of the working fluid.
U.S. Pat. No. 5,239,356 describes a CNC having multiple air flow paths.
An additional shortcoming of prior art CNC devices is that conventional CNCs require a considerable amount of time to reach steady state from a cold start, typically on the order of 40 minutes to one hour before particulate monitoring can begin.
What is needed is a condensation nucleus counter which exhibits a stable .DELTA.T about a desired setpoint temperature, in order to ensure accurate particle counts. There is also a need for a CNC which exhibits a shortened warm-up time so that particulate monitoring can begin without requiring an operator to wait for the system to stabilize. There is need to provide a CNC which can detect and count different ranges of particle sizes without having to employ filtering devices such as diffusion batteries and inertial impactors, and which can quickly adjust to the new settings.